1. Field of the Invention
This invention relates to an active matrix type liquid display apparatus, for example, a plasma address liquid crystal display apparatus having a flat panel structure comprised of a display cell and a plasma cell superposed together. More particularly, it relates to a technique of improving the view angle dependency of the display cell having the liquid crystal as a electro-optical device and further to a technique of moderating the registration tolerance between the display cell and the plasma cell.
2. Description of the Related Art
Among active matrix system liquid crystal display devices (LCDs), there is known a TFTLCD employing a thin-film transistor (TFT) as a pixel driving switching device and a plasma address liquid display (PALC) employing a plasma device. The active matrix system liquid crystal display devices are widely used for a personal computer or a portable monitor for navigation in order to save space and power consumption. In particular, development of the plasma address liquid display is under way as a large-sized display of a flat-panel structure to take the place of the CRT, as disclosed for example in Japanese Laying-Open Patent H-1-217396 or in Japanese Laying-Open Patent H-4-265931. The plasma address liquid display has a flat-panel structure comprised of a display cell and a plasma cell interconnected via an intermediate sheet in-between. The plasma cell is used for addressing the display cell. In the display cell, liquid crystal is prevalently used as a electro-optical device. The plasma cell has plural discharge channels extending in the column direction. The display cell has a electro-optical device such as a liquid crystal and plural signal electrodes in the column direction. A pixel matrix is defined at each point of intersection between the rows of the discharge channel and columns of the signal electrodes.
Conventionally, a display cell routinely uses the TN mode which uses a nematic liquid crystal in a twist orientation as a electro-optical device. However, the TN mode exhibits significant viewing angle dependency, such that the contrast or luminance is varied depending on the angle with which the screen is viewed (viewing angle) to render the display cell difficult to use as a large-sized display. A powerful technique for improving the viewing angle dependency of the display cell employing the liquid crystal is disclosed in Japanese Laying-Open Patent H-6-301015 or in Japanese Laying-Open Patent H-7-120728. The electro-optical device used in this display cell is a set of finely partitioned liquid crystal areas, each of which has orientation controlled with axial symmetry. This axial symmetry helps improve viewing angle dependency by axial symmetry of the liquid crystal orientation significantly.
A liquid crystal display device, which combines a display panel exploiting the axial symmetry orientation technique for a liquid crystal with a plasma cell, has already been developed, as disclosed in Japanese Laying-Open Patent H-9-197384. With this system, a liquid crystal area (liquid crystal domain) having orientation controlled axis-symmetrically is formed in association with a physical pixel positioned at a point of intersection of a row of discharge channels and a column of signal electrodes. This liquid crystal domain represents an optical pixel. An area formed by superposition of the physical pixel on the plasma cell side and the optical pixel on the display cell side represents an effective aperture of the pixel. For assuring an aperture area matched to a design value, it is necessary to realize correct registration between the physical pixel and the optical pixel. However, with increasingly high pixel density of a large-sized display and increasingly high definition, correct registration is becoming more and more difficult. On the other hand, there is a certain gap between the physical pixel and the optical pixel. The result is that, if a screen is viewed from an oblique direction, there is produced parallax between the physical pixel and the optical pixel. Due to this parallax, if the screen is viewed from an oblique direction, the physical and optical pixels are not overlapped in the worst case thus occasionally closing the effective aperture.